We've had great success with Windows 2000, and I have to say that XP has gone near-flawlessly so far. We have had ONE piece of software that had an issue with the installer program the vendor used...
Windows 2000 has software compatibility properties that XP has, you can run them in the context of Win95, WinNT SP5, etc. This might be something that is added with SP2, but I know for sure you can add it with the Windows update with no issues.
We currently have over 1000 machines and laptops operating on Windows 2000. There are greater than 50 different software titles that are operating successfully on Windows 2000 including Office 2000/XP, Lotus Notes, AS400 and mainframe tools and client software, programming software for every language you could think of, modeling software... the list goes on and on. Another 20 applications which this company produces also operate in 2000 without issues. Software compatibility really is dependant on the vendor, if the software is certified with 2000, then it's going to work.
We still have another 3000 Windows NT4 machines as well, so co-existance of the two in an environment is not an issue.
Windows XP is looking GREAT so far. It DOES require a faster machine, as with any new operating system. You will have issues where your vendors arent quite ready for XP support, but that shouldnt be such a big issue with real companies who mean business. If the company hasnt yet realized they have to keep on top of things, and they havent yet come to understand Microsoft's OS release plan, then I would seriously re-consider my business arrangement with them.
Windows ME and 98SE are home software. Networking and stability are not the selling attributes of these products (ME is just an updated version of 98SE anyway) and should not be considered for business purposes.
These are just my opinions and observations, nothing less and nothing more.